1. Field of the Invention
The present invention relates to an optical communication system particularly suitable for use in gathering data from a number of sensors. Sensors of the type the present invention is concerned with are used, for example, in industry for process control, pollution monitoring and chemical/biological analyses. The system might also be used with domestic sensors, such as smoke detectors or burglar alarms.
The use of optical rather than non-optical wired networks for such sensors potentially offers a number of advantages, including immunity from sparking in a flammable environment, and immunity to electro-magnetic interference (EMI). However because of their characteristics of more or less continual data output at low data rates, sensors are not generally suitable for connecting directly to conventional optical telecommunications networks. It would be desirable to provide a network particularly suitable for sensor data but which also carries diverse other types of traffic.
2. Related Art
It has previously been proposed to provide a system using an optical fibre network having a bus-type topology to communicate data from a plurality of sensors to a head-end station. Each branch of the network is connected to the bus by a wavelength selective tap, so that each sensor communicates data on a different characteristic wavelength. Such a system is disclosed in "Wavelength Division Multiplexing in Optical Fibre Sensor Systems and Networks", J. M. Senior et al, OPTICS AND LASER TECHNOLOGY, Vol. 22, No. 1, 1st Apr. 1990. This network is primarily designed for one-way traffic from sensors to the head-end station. There are however situations where it would be desirable to provide two-way data communication on the network. In many systems for use in industrial processes, there will be associated with each sensor an actuator, such as a valve, switch or other control device, which is responsive to control signals received over the network so as to provide a control loop from the sensor via the head-end to the actuator. These control signals may be processed by a local computer at the actuator. In the context of the present specification, the term "actuator" also encompasses any other device responsive to optical signals received over the network, including the receive side of a telephone, where the two-way system is used to integrate, e.g. voice telephony with the carrying of sensor data.
EP-A-0206901 discloses a network in which a demultiplexer splits a broadband optical signal into different wavelength components and outputs each component on a different optical fibre going to a respective sensing or control device.